Can't add custom attribute to <desc> element

[sixers]

AFAIU, this parser is what HTMLRewriter in CloudFlare workers uses under the hood.
I have a simple worker which transforms the HTML received as request body and adds a new attribute to each node. It seems that there is a bug, and the attribute change is not applied to <desc> element.

Example:

class EnumerationElementHandler {
    constructor() {
        this.counter = 0;
    }
    element(element) {
        const c = this.counter++;
        element.setAttribute('data-custom', c.toString())
    }
}

const rewriter = new HTMLRewriter().on("*", new EnumerationElementHandler())

export default {
    async fetch(request) {
        return rewriter.transform(new Response(request.body, {status: 200}));
    },
};

Now, I send a POST request with the following body:

<div><svg viewBox="0 0 460 271.2" width="70px"
    height="41px" aria-labelledby="title desc">
    <title>
    <span>X</span></title>
    <desc><span>Y</span>
    </desc>
    <path id="flare" fill="#fff"
    d="M370.9,150.2l-40.7-24.7c-0.6-0.1-4.4,0.3-6.4-0.7c-1.4-0.7-2.5-1.9-3.2-4c-3.2,0-175.5,0-175.5,0    v91.8h225.8V150.2z"></path>
    
</svg></div>

Output:

<div data-custom="0"><svg viewBox="0 0 460 271.2" width="70px" height="41px" aria-labelledby="title desc" data-custom="1">
    <title data-custom="2">
    <span data-custom="3">X</span></title>
    <desc><span data-custom="4">Y</span>
    </desc>
    <path id="flare" fill="#fff" d="M370.9,150.2l-40.7-24.7c-0.6-0.1-4.4,0.3-6.4-0.7c-1.4-0.7-2.5-1.9-3.2-4c-3.2,0-175.5,0-175.5,0    v91.8h225.8V150.2z" data-custom="5"></path>
    
</svg></div>

As you can see, data-custom attribute is added to every node but <desc>.